Hydrogen sulfide is a well-known and potent metabolic toxin that poisons mitochondrial oxidative phosphorylation at micromolar concentrations. An accumulating body of evidence, however, suggests that sulfide may exert other toxic effects in humans, and that chronic exposure to sulfide concentrations that are currently considered safe may actually have long- term effects on central nervous system functions. For the past decade, we have been studying invertebrates that are evolutionarily adapted to sulfide concentrations that would be lethal to other animals. Recently, this research has uncovered a number of toxic sulfide effects that are independent of oxidative phosphorylation and that likely have parallels with the pathological effects of sulfide in animals. With this foundation, we will now expand our research scope to focus on the effects of sulfide across a broad range of animals. The objective of the research program of this application for competing continuation of MBRS funding is to characterize the cellular and molecular mechanisms of hydrogen sulfide toxicity, with a long-term objective of identifying the pathology of sulfide toxicity, with a long-term objective of identifying the pathology of sulfide toxicity in humans. We propose to 1) define the mechanism by which sulfide inhibits muscle contraction, 2) determine whether sulfide is produced endogenously in animal tissue and has synergistic interactions with nitric oxide, 3) investigate the long-term effects of sulfide emphasizing induction of apoptosis and degenerative changes in cell and tissue ultrastructure, and 4) initiate investigations of the toxic effects of sulfide on nerve function. In achieving these objectives, we will provide information essential for understanding the effects of sulfide exposure on humans, for developing treatment strategies for sulfide poisoning, and for redefining occupational and public health exposure limits. Our research effort has consistently involved underrepresented minority undergraduate and graduate students, and numerous aspects of this project will provide excellent opportunities for MBRS students to formulate hypotheses and realize their research goals while training on modern equipment. This hands-on training, followed by publication and presentation of their research results will encourage and enable them to successfully pursue a career in the biomedical sciences.